What was found
(CAN) WHILE TROUBLESHOOTING FOR A PNEUMATIC BLEED AIR PROBLEM, NOTED A LT BLEED AIR DUCT ASSY, PN 129-910033-15 CRACKED APPROXIMATELY 75 PERCENT OF THE WAY AROUND THE DUCT JUST AFT OF THE FLANGE THAT ATTACHES THE DUCT TO THE FIREWALL. PART REPLACED WITH NEW. THEIR WAS NO EXCESSIVE LOADS ON THE PART, ALIGMENT WAS FINE, THAT WOULD CAUSE THE DUCT TO CRACK. THE CRACK WAS DIFFICULT TO SEE AS THE DUCT IS COVERED WITH INSULATION AND ALUMINUM TAPE FROM THE FACTORY TO SHIELD FROM THE HIGH BLEED AIR TEMPERATURES. DURING THE TROUBLESHOOTING PROCESS SHOP AIR WAS SUPPLIED TO THE BLEED AIR LINES. THIS IS HOW THE CRACK WAS DISCOVERD.